Can I use Defender with another antivirus?

Defender enters passive mode when another AV is primary. It can still do periodic scans but real-time protection is handled by the other product.

How do I fix error 0x80070643?

This usually means the Defender definition update package is corrupted. Download definitions manually from Microsoft's website and install them.

Windows Defender Errors — Antivirus Not Working or Updating

Errorsecurity

Overview

Fix Windows Defender errors including service not starting, definition update failures (0x80070643), real-time protection disabled, and scan failures.

Key Details

Windows Security / Defender may show 'Your virus & threat protection is managed by your organization'
Error 0x80070643: definition update failed during Windows Update
Real-time protection toggled off and grayed out usually means Group Policy or third-party AV conflict
Error 0x80073b01: Windows Defender service failed to start
Defender cannot run alongside most third-party antivirus products (except in passive mode)

Common Causes

Third-party antivirus installed, disabling Defender automatically
Group Policy or registry settings disabling Windows Defender
Corrupted Windows Defender definitions or program files
Windows Security service (WinDefend) disabled or crashed
Malware specifically targeting and disabling Windows Defender

Steps

1Uninstall any third-party antivirus completely (use their removal tools for full cleanup)
2Check Group Policy: gpedit.msc > Computer Config > Admin Templates > Windows Components > Windows Defender > ensure 'Turn off' is Not Configured
3Reset Defender via PowerShell: Get-AppxPackage Microsoft.SecHealthUI -AllUsers | Reset-AppxPackage
4Manually update definitions: run elevated cmd > cd "%ProgramFiles%\Windows Defender" > MpCmdRun.exe -SignatureUpdate
5Check WinDefend service: services.msc > Windows Defender Antivirus Service should be Running and Automatic